The present invention generally relates to thermal energy storage systems and, more particularly, to systems utilizing phase change materials.
Vehicles and electronic devices often require a means for storing or absorbing large amounts of heat. The heat may be stored or absorbed at temperatures that make it available for later dissipation.
A thermal energy storage system may transfer heat via a heat transfer fluid, such as ethylene glycol/water, to a thermal energy storage material. Thermal energy storage systems store energy by heating, melting, or vaporizing the thermal energy storage material. The energy becomes available as heat when the process is reversed and the material is cooled, solidified, or liquefied. In some systems heat may be stored by raising or lowering the temperature of a material without changing its phase: This is known as sensible heat storage. In some systems heat may be stored by causing a phase change in a material: This is known as latent heat storage. Both sensible and latent heat storage may occur in the same material.
The use of phase change materials (PCMs) in thermal energy storage systems is known in the art. PCMs can store thermal energy, latent heat, by transitioning from solid to liquid or from liquid to vapor. As a solid PCM is heated its temperature increases until it reaches its melting temperature. At its melting (transition) temperature, the PCM remains in the solid phase while it absorbs a fixed amount of heat known as the “latent heat of fusion” (also called the “latent heat of crystallization”). Once the PCM absorbs the latent heat of fusion, it changes phase from solid to liquid. To reverse the process heat is removed from the PCM. Its temperature decreases until it reaches the PCM's fusion temperature. The PCM stays in the liquid phase until it releases an amount of heat equal to the latent heat of fusion. As the PCM continues to loose heat, the PCM changes from the liquid phase to the solid phase. Absorbing and releasing the latent heat of fusion at a characteristic transition temperature allows the PCM to store relatively large amounts of heat without having to be raised to correspondingly high temperatures. Thus a given mass of PCM can store relatively more heat at a given temperature than the same mass of a single-phase material using only its sensible heat capacity. PCMs have been used in many industries for heat storage. PCMs have been packaged in containers such as tubes, shallow panels and plastic bags or encapsulated as self-contained grains.
U.S. Pat. No. 5,687,706 discloses a heat storage system utilizing a PCM. The described system comprises a packed-bed slurry of encapsulated PCM pellets with a heating element. This system is specifically designed to inhibit vertical thermal convection and can only be used in a vertical configuration making it unsuitable for some applications.
U.S. Pat. No. 6,059,016 describes a thermal energy storage and delivery system for delivering thermal energy to both a passenger compartment of a vehicle and a component, such as a battery. In some embodiments of the disclosed system, the bulk mass of the PCM is in direct contact with both a vehicle hot fluid and a vehicle cold fluid with the fluid flow depending on electro-mechanical control systems. In some embodiments of the disclosed system, the PCM is encapsulated and used in a cross-flow tube-and-shell arrangement. The PCM encapsulation tubes require corrugations to mitigate expansive loading which reduces tube packing density for some applications. For some applications a reduction in tube packing density results in a corresponding reduction in heat absorption by the storage system.
U.S. Pat. No. 4,807,696 provides a PCM heat storage system. The system comprises a housing defining a chamber and having an inlet and an outlet. An aluminum honeycomb matrix is positioned within the chamber and filled with macrocapsules. The macrocapsules have a PCM core and an encapsulating outer shell. The honeycomb matrix divides the flow of heat transport fluid into separate flow streams channeling the flow streams from the inlet to the outlet. Although this system may be used to store thermal energy, the reduction in PCM packing density due to the honeycomb matrix results in a system efficiency reduction for some applications. For some space-limited applications the described system may not provide sufficient heat storage and a more densely packed PCM arrangement may be desired.
Thus there is a need for an improved thermal energy storage system: A system is needed that is not limited to a vertical configuration, a storage system is needed that mitigates expansive loading without the need for encapsulation tube corrugations; and, a thermal energy storage system is needed wherein PCM packing density is increased.